<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button>按钮1</button>
    <button>按钮2</button>
    <button>按钮3</button>
    <script>
        let btns = document.querySelectorAll('button');

        for(let i=0;i<btns.length;i++){
            btns[i].onclick = function(){
                console.log(i);
            }
        }

        // console.log(i);
    </script>
</body>
</html>